Thermal expansions of NpO2 and some other actinide dioxides

Thermal expansions of the stoichiometric actinide dioxides (ThO2, UO2, NpO2 and PuO2) were investigated between room temperature and 1300 K using a high temperature X-ray diffraction method. The lattice parameter of NpO2 at high temperatures was expressed as aT (pm)=542.03+4.28 x 10-3T+9.07 x 10-7T2-1.36 x 10-10T3. Based on excellent reproducible data, thermal expansion of NpO2 was determined. Thermal expansions of the other actinide dioxides showed good agreement with the respective recommended literature values. The linear thermal expansion coefficients (α) of actinide dioxides calculated at 1200 K were in inverse relation to their melting points. At room temperature, however, the α value of UO2 was found to be higher than those of the other actinide dioxides and this result was discussed. (orig.)